1. Field of the Invention
The present invention concerns a tomosynthesis apparatus as well as a method to operate a tomosynthesis apparatus.
2. Description of the Prior Art
Tomosynthesis is an x-ray-based 3D imaging method which is particularly suitable for use in mammography. Through continuous development it is sought to generate x-ray images with high significance in order to differentiate benign variations from malignant variations and to reduce the number of incorrect findings, i.e. the number of suspicious findings that are caused by non-malignant variations and the number of undiscovered malignant tumors.
An example of a tomosynthesis apparatus and operating method is described in DE 10 2006 024 413 A1.
In conventional mammography, a two-dimensional single image of the compressed breast is generated. In tomosynthesis of the breast, single images (projections) are respectively acquired from different directions, from which a tomosynthetic image data set is generated. The projections are used in a calculation to form tomosynthetic 3D x-ray image using image reconstruction methods. In this way structures can also be identified and examined that would occlude one another in a conventional mammogram that consists only of one x-ray image acquired from a single projection direction.
To acquire a tomosynthesis image data set, the examination subject (for example the breast) is irradiated from a number of different directions, to acquire the individual projections. The different directions from which the examination subject is exposed to acquire the individual projections are characterized by what are known as tomosynthesis angles. The different radiation directions or tomosynthesis angles are achieved by panning the x-ray source around the examination subject within a limited angle range, for example by ±20° starting from a center position. The x-ray source can thereby ensue in a plane perpendicular to the plane of the x-ray detector, as is typical in the tomosynthesis examination of the breast. The movement of the movement of the x-ray source and thus its x-ray focus, typically ensues essentially along a straight line or along a circular arc. In what is known as circular tomosynthesis, the x-ray source is moved in a plane oriented parallel to the detector plane. The x-ray focus thereby follows in a circular path, for example.
The x-ray detector (normally a flat panel detector) that receives (detects) the x-ray beam emanating from the x-ray source remains essentially stationary during the movement of the x-ray source, meaning that the x-ray detector actually remains stationary or is entrained only slightly in the direction opposite the movement of the tube.
The x-ray beam emanating from the x-ray source is limited by a collimator diaphragm so that the exposure of the flat panel detector is optimal. The exposure is considered as optimal when the entire detector area of the flat panel detector is utilized, i.e. is exposed. The radiation field at the location of the detector thus essentially corresponds to the detector area. The situation of the x-ray beam having a cross-section that, at the location of the flat panel detector, either exceeds the dimensions of the flat panel detector, or that has a portion that does not actually strike the flat panel detector, is referred to herein as an “overexposure.”
As used herein, therefore, the term “overexposure” does not mean an unwanted darkening of the image, due to an excessively high radiation intensity or an excessively long exposure duration.
In conventional x-ray apparatuses, the collimator diaphragm is often placed or plugged by hand into a tray or a slot provided for the diaphragm. Normally a small metal lamella (thin plate) into which a fixed collimator diaphragm aperture is punched or milled, is used as the collimator diaphragm.
The collimation of the x-ray beam in tomosynthesis in which the x-ray source is moved relative to the detector has conventionally been achieved in a less than satisfactory manner. Both an insufficient exposure of the detector surface and overexposure of the detector have been observed from different tomosynthesis angles.